Inductively activated control and protection circuit for refrigeration systems

ABSTRACT

The present invention involves an inductively activated silicon switch for a control and protection circuit for motors and compressors. The circuit includes a logic gate for combining both motor control and protection functions. The logic gate has inputs from a thermostat, a compressor shell temperature circuit, a fan overheat detection circuit, a motor start relay circuit, and motor load sensing circuits. The logic gate output is connected to an oscillator for inductively activating the power output stage, as well as a start relay, a compressor shell temperature hysteresis circuit, and a fan timer circuit connected to the fan control circuit. The control circuitry further includes an over-ride circuit for manually allowing a limited number of immediate restart attempts.

This is a division of application Ser. No. 08/131,420, filed Oct. 4,1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to control and protection circuitry forrefrigeration systems. More particularly, the field of the inventioninvolves circuitry for activating and deactivating a compressor and afan motor.

2. Description of the Related Art

Motor protection devices generally include electro-mechanical or solidstate electronic devices for protection and control of motors orcompressors. Conventional motor protection devices seek to regulate thecurrent drawn by the compressor motor under various loads andconditions. By limiting the amount of current provided to the compressormotor, conventional motor protection devices protect the compressor'swindings from damaging effects of high currents and high temperatures.

For example, one conventional motor protection device is a snap discplaced in series with the windings of the compressor motor. The snapdisc is composed of bi-metallic layers which are in physical contactwith the contact points which close the circuit. Typically, a resistiveheating element which heats the bi-metallic layers is connected inseries with contact points such that when the heat generated bycompressor current passing through the resistive element exceeds theallowable threshold, the different metals of the bi-metallic layerexpand at different rates, causing the disc to bend. This bending of thedisc breaks the connection to the contact points, thereby opening thecircuit to the compressor motor. Another arrangement involves placingthe snap disc device in close proximity to the compressor motor so thatthe snap disc device may open and close in response to the temperatureof the compressor motor.

Several problems may occur with a conventional snap action bi-metallicmotor overload protector. One problem with the snap disc device is thatthe overload condition may be detected only after a significant amountof time has passed since the condition originally developed. During thislag time, significant damage to the windings of the motor can occur.Also, conventional snap disc overload protection devices are generallyimprecise and non-dynamic. For instance, the temperature and current setpoints of a snap disc cannot account for different environmental ormotor loading conditions. Finally, once the snap disc has opened thecircuit to the motor windings, the restoration period of the bi-metallicdevice is typically excessively lengthy.

Additional motor protection devices include solid state electronicdevices which control the power delivered to the compressor motor. Incontrast with the electro-mechanical snap disc devices, solid stateprotection devices have the advantages of precision, reliability, andself-regulation. Generally, a conventional electronic protection deviceincludes a thermostat to sense ambient and internal compressortemperatures, control logic responsive to inputs and which controls thecorresponding outputs, and solid state power components which are usedto apply power to the compressor motor. For instance, thermostats usingthermistors as temperature sensing inputs to an electronic motor controlcircuit are disclosed in U.S. Pat. No. 5,231,848, "REFRIGERATOR COLDCONTROL", issued Aug. 3, 1993, which is assigned to the assignee of thepresent invention, the disclosures of which are explicitly incorporatedby reference.

Prior art motor protection devices typically include power output stageswhich regulate the application of power to the compressor motor. Theoutput of the control logic circuit drives the output power stage,either by direct electrical connection to the output stage or byindirect magnetic coupling through a relay. Both techniques offersignificant advantages in accuracy, reliability, and precision overelectro-mechanical methods for controlling and protecting compressormotors.

However, circuits which directly couple the control logic circuit to thepower output stage suffer from problems associated with noise inducedinto the control logic circuit from the high current flow of the poweroutput stage. In order to eliminate such problems, conventional solidstate control circuits utilize a relay to control the activation gate ofa solid state switch element, such as a SCR or TRIAC. While the use of arelay offers the benefit of electrical isolation of the control logiccircuit and the power output circuit, the use of relays in compressormotor protector circuits may also be problematic. For instance, underhigh temperature conditions the metallic contacts of the relay may meltdown and permanently close due to excessive compressor temperatures.Furthermore, the physical contacts within the relay are subject todamage from repeated wear, corrosion, metal fatigue, or other physicallydegrading conditions.

What is needed is a compressor motor protection device which is not assubject to noise problems or physical degradation as conventional motorprotection devices.

Also needed is a motor protection device which is more accurate andprecise than conventional electro-mechanical protection devices.

SUMMARY OF THE INVENTION

The present invention combines the control and motor protectionfunctions into a circuit which inductively activates a solid stateswitch which gates electrical current to the compressor motor. Thecircuitry of the present invention provides precise control of thecompressor motor while electrically isolating the power switching fromthe more sensitive control and sensing circuitry. Also, the presentinvention combines the compressor and fan control functions, allowingfor more efficient system control by coordinating the operation of thecompressor and fan.

The circuitry of the present invention provides many performancefeatures in an efficient and economical arrangement. For example, therelatively quick response of the circuitry limits the duration of alocked rotor condition versus conventional circuitry using snap discs orrelays. Also, the circuitry checks against low line voltage or lowtemperature ambient, and disables the compressor motor in the event ofsuch a condition. Also, by selecting appropriately sized electricalcomponents, the control circuitry provides a selectable temperaturehysteresis.

Upon start-up, the circuitry of the present invention includes a startrelay for decreasing the equivalent impedance of the run capacitorduring motor start-up. Also, an optional start-up delay avoidancecircuit is provided to allow, for a limited number of tries, a manualover-ride of the locked rotor protection circuitry. However, the circuitprevents an excessive number of such over-ride attempts.

The present invention, in one form, is a refrigeration system forcooling a chamber, a compressor having a motor adapted for connection toa power supply, and a control circuit for controlling the activation ofthe compressor motor. The circuit includes a solid state switch forelectrically coupling the power supply and the compressor motor. Thesolid state switch includes an activation gate which opens and closescurrent flow through the solid state switch. The system further includesan inductive coupling for inducing a current on the activation gate ofthe solid state switch to actuate the activation gate and therebyprovide power to the compressor motor.

The refrigeration system further includes an oscillation deviceoperatively associated with the inductive coupling for driving the solidstate switch, and a device for sensing the operating condition of thecompressor motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and objects of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic diagram of the components of the refrigerationsystem of the present invention;

FIGS. 2A and 2B form a schematic circuit diagram of the compressor motorprotection circuit of the present invention;

FIG. 3 is a schematic circuit diagram of a manual delay avoidancecircuit for the compressor motor protection circuit;

FIG. 4 is a schematic circuit diagram of a portion of a secondembodiment of a compressor motor protection circuit;

FIG. 5 is a schematic circuit diagram of a second embodiment of a solidstate power control circuit; and

FIG. 6 is a schematic circuit diagram of an alternative embodiment ofthe output stage.

corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates preferred embodiments of the invention, in several forms,and such exemplifications are not to be construed as limiting the scopeof the invention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments disclosed below are not intended to beexhaustive or limit the invention to the precise forms disclosed in thefollowing detailed description. Rather, the embodiments are chosen anddescribed so that others skilled in the art may utilize their teachings.

The present invention relates generally to compressor motor controllersfor refrigeration systems shown in FIG. 1. Controller 7 is electricallyconnected to thermostat 6, compressor 8, and fan 9. Thermostat 6 islocated within chamber 11 to detect the temperature within the chamber,and provide appropriate information to controller 7. Controller 7activates and deactivates compressor 8 and fan 9 in order to control thetemperature in chamber 11.

FIGS. 2A and 2B show a schematic circuit diagram of controller 7including thermostat 6. The circuit includes NAND gate G1 for combiningboth motor control and protection functions, which has inputs fromthermostat 20, compressor shell temperature circuit 21, fan overheatdetection circuit 28, over-ride circuit 23, and motor load sensingcircuits 24, 25, and 26. The system outputs, connected to output pin 3of NAND gate G1, include oscillator 30 connected to power output stage31, motor start relay 29 connected to TRAIC3 and TRIAC1, compressorshell temperature hysteresis circuit 22, and fan controller 27 which isconnected to fan activation circuit 32.

NAND gate G1 accepts inputs from the system's sensors, and controls thecompressor motor through resistor R24 and transistor Q9. Pull-upresistor R9 maintains input pin 1 of NAND gate G1 at a high voltagelevel unless an open-collector device or other input device pulls thevoltage at pin 1 low.

Solid state power control to compressor motor M is governed by NAND gateG1, transistor Q9, oscillator 30, and power output stage 31. Poweroutput stage 31 utilizes solid state switches, such as SCR1 and SCR2, oralternatively TRIAC4 shown in FIG. 5, to perform solid state powerswitching to compressor motor M.

Under normal conditions, oscillator 30, composed of NAND gate G3,feedback resistor R4, and charging capacitor C6, generates highfrequency oscillations of a period proportional to the RC time constantwhich is the product of the values of resistor R4 and capacitor C6. Theoutput waveform produced by oscillator 30 feeds power output stage 31.

Transistors Q8 and Q3, resistor R25, and capacitor C5 form acomplementary push-pull amplifier which is connected to the primary sideof transformer T1. The push-pull amplifier is used to conduct the signalproduced by oscillator 30 during both positive and negative cycles ofoscillation.

The secondary side of power output stage 31 is composed of transformerT1, diodes D11 and D12, and reverse blocking triode thyristors SCR1 andSCR2. The high frequency periodic pulses appearing on the primary sideof transformer T1 control the gate voltages applied to SCR1 and SCR2 onthe secondary side of transformer T1. SCR1 and SCR2, in inverse-parallelarrangement, control the application of power to compressor motor M.Specifically, when diode D11 is forward biased, current travels into thegate of SCR2, thereby activating SCR2 for conduction. Likewise, whendiode D12 is forward biased, current travels into the gate of SCR1,thereby activating SCR1 for conduction. Since the frequency of theoscillations produced by oscillator 30 are much higher than the 60 hzline frequency, the inverse-parallel arrangement of SCR1 and SCR2delivers AC power to compressor motor M utilizing the positive ornegative cycles of the AC line voltage, as long as oscillator 30 isrunning.

The circuit diagram of FIG. 5 shows a second embodiment of power outputstage 31. Referring to FIG. 5, secondary winding N2 provides the gatevoltage to TRIAC4 through resistor R67. In this configuration, TRIAC4 isused in place of SCR1, SCR2, and diodes D11 and D12. The use of solidstate switches such as thyristors, either SCR's or a TRIAC, forcontrolling the provision of electrical power to compressor motor Mensures that compressor motor M is turned off at the next zero crossingof motor current. With prior art snap discs, motor current is typicallyinterrupted at a point of relatively high current, thereby generatinghigh voltage in the motor windings.

Deactivation of compressor motor M is achieved by stopping theoscillation of oscillator 30. While oscillator 30 begins running whenpower is applied to the circuit, transistor Q9 controls the subsequentoperation of oscillator 30. When the base voltage of Q9 through resistorR24 is low, transistor Q9 turns off, and oscillator 30 drives poweroutput stage 31 thereby activating compressor motor M. When the basevoltage of transistor Q9 is high, Q9 conducts from collector to emitter,holding the voltage of input pin 12 of NAND gate 3 low, therebysuspending oscillator 30, which deactivates compressor motor M.

In order to monitor the operation of compressor motor M, voltage sensingand current sensing techniques are employed by the present invention.Voltage sensing is a technique used to determine the present load oncompressor motor M. When compressor motor M is in a running state, asthe compressor load increases, the voltage across the auxiliary windingdecreases. Input pin 2 of NAND gate G1 is connected to voltage referencebranch 25, voltage sensing branch 24, and over-ride circuit 23 in orderto sense the operating condition of compressor motor M.

Voltage sensing branch 24 includes resistors R7 and R1, and diode D6which are connected to the auxiliary winding of compressor motor M.Compensating network 26 includes resistor R26 connected to the cathodeof zener diode D1. The anode of zener diode D1 is connected to DCground, referred to as supply reference (SR). Reference branch 25 iscomposed of resistor R14 and diode D5, whose cathode is connected to ACcommon.

Sensing branch 24, in conjunction with reference branch 25 andcompensating network 26, senses the voltage level across the auxiliarywinding of compressor motor M. Under light loading conditions, thevoltage across the auxiliary winding of compressor motor M is large,therefore the voltage at input pin 2 of NAND gate G1 is high. However,as the load on compressor motor M increases, the voltage across theauxiliary winding decreases. When this winding voltage declinessufficiently, the voltage @pin 2 reaches the negative threshold of inputpin 2, causing the output of NAND gate G1 to jump to a high state,thereby inhibiting oscillator 30, which deactivates compressor motor Mthrough power output stage 31. In this manner, compressor motor M isdeactivated when the motor load exceeds allowable limits. NAND gate G2operates with capacitor C3 and resistor R6 to provide a motor off cycletimer, which latches gate G1 off for about 47 seconds after compressormotor M is deactivated because of excessive motor load.

FIG. 4 shows a schematic diagram of a second embodiment of the solidstate motor control. This circuit configuration implements currentsensing techniques to determine motor loading. Referring to FIG. 4,transformer T2 has primary winding N_(p) connected in series withcompressor motor M. The changing load current traveling through theprimary winding of transformer T2 induces a corresponding voltage acrossthe secondary winding, N_(s), of transformer T2. This changing secondaryvoltage feeds the base of transistor Q60 through resistor R65. As theload current increases, the base voltage of transistor Q60 alsoincreases relative to the DC supply ground. When the load current hasreached the maximum allowable level, transistor Q60 is pulsed on andconducts current through resistor R60 such that the voltage at input pin2 to NAND gate G1 is pulled low, thereby setting the output voltage ofNAND gate G1 high. As a result, oscillator 30 is suspended from drivingpower output stage 31, thereby deactivating motor M. Once output pin 3of gate G1 has jumped to a high state, it is necessary to latch inputpin 2 in a low state until the motor protector timer, which comprisestransistor Q12, capacitor C3, resistors R₅, R₆ and gate G2, produces anover-ride pulse at pin 2. This latch function is formed by resistor R64and transistor Q60. Thus, transistor Q60 performs dual functions ofcurrent sensing and off-cycle latching.

Following the deactivation of compressor motor M, the motor protectorproduces an over-ride pulse to charge capacitor C2 to a voltageexceeding the positive threshold of G1 at input pin 2. If motor loadcurrent then remains below the trip level as determined by the voltagegenerated in winding N_(s) of transformer T2, and by the thresholdcircuitry consisting of transistor Q60, resistors R65, R62, and R63,compressor motor M continues to be energized. Resistors R62 and R63 forma voltage reference at the emitter of Q60 relative to the DC powersupply ground.

In addition to sensing the load conditions of compressor motor M, otherinputs to the motor protection system provide additional sensing forcontrol of compressor motor M and fan 9.

Thermostat 20 is composed of negative temperature coefficient NTCthermistor R19, potentiometer R20, PNP transistor Q5, and resistors R16,R17, R3. NTC thermistor R19 and potentiometer R20 form a resistivedivider which feeds the base of transistor Q5. Potentiometer R20establishes the temperature set-point within chamber 11. As thetemperature within chamber 11 increases above the set-point, theresistance of NTC thermistor R19 decreases, thereby deactivating Q5 andQ6. Transistor Q6 controls the voltage of pin 1 of NAND gate G1.

However, when the temperature within chamber 11 decreases, theresistance of NTC thermistor R19 increases, thereby turning transistorQ5 and Q6 on, which generates a low voltage at pin 1 of NAND gate G1.When the input voltage of pin 1 of NAND gate G1 is low, the output pinof NAND gate G1 deactivates oscillator 30, thereby deactivatingcompressor motor M.

Compressor shell temperature circuit 21 and compressor temperaturehysteresis circuit 22 also provide control inputs to NAND gate G1. PTCthermistor R2 may be located on the exterior surface of compressor 8 todetect the compressor shell temperature. A resistive divider is formedby PTC thermistor R2 and resistor R23. Transistor Q7 is off under normaltemperature conditions keeping R12 oat of the divider network. As thetemperature of the compressor increases, the resistance of PTCthermistor R2 also increases, thereby decreasing the voltage presentacross resistor R23. When the compressor temperature reaches the maximumallowable limit governed by R23 and R2, diode D2 is forward biased andpulls the voltage of input pin 1 of NAND gate G1 low, therebydeactivating compressor motor M.

Additionally, when the output voltage of NAND gate G1 is high,transistor Q7 of temperature hysteresis circuit 22 turns on, introducingresistor R12 in parallel with resistor R23 of the lower element of theresistive divider. By reducing the lower element of the equivalentresistance of the resistive divider of compressor shell temperaturecircuit 21, transistor Q7 and resistor R12 ensure that the compressorcools to a sufficiently low temperature before a restart attempt may bemade.

Circuit 23 provides an over-ride input into NAND gate G1 to control thestarting of compressor motor M. Over-ride circuit 23 is only activeduring motor starting, and includes transistors Q11 and Q12, resistorsR11 and R5, and NAND gate G2. Note that the emitter of transistor Q12 isnot connected, and transistor Q12 functions as a diode having acharacteristic of very low current leakage.

Fan overheat detection circuit 28 is composed of negative temperaturecoefficient thermistor R49, resistor R50, and NAND gate G7. NTCthermistor R49 may be located in thermal contact with the fan motor offan 9. As the temperature of the fan motor increases, the resistance ofNTC thermistor R49 decreases, thereby increasing the voltage presentacross resistor R50 and at input pin 2 of NAND gate G7. Therefore, whenthe fan motor temperature exceeds the limit established by thermistorR49 and resistor R50, fan 9 is disabled through NAND gate G7, NAND gateG6, transistor Q23, resistor R27, and TRIAC2.

A manually operated delay-avoidance or over-ride circuit is shown inFIG. 3. This circuit allows the user to start the compressorimmediately, avoiding waiting until the expiration of the motorprotector "off" period. The delay-avoidance circuit of FIG. 3 includes anormally open momentary push-button switch SW1, resistors R68-R70,transistors Q61 and Q62, diode D16, and capacitor C15. When switch SW1is momentarily closed, capacitor C15 charges, and transistor Q61 turnson. The emitter of Q61 sets the input voltage of NAND gate G2 to a highlevel, causing the output of gate G2 to drop low, thereby chargingcapacitor C2 to a high level and allowing the start-up of compressormotor M.

Motor start relay 29 is operative during starting to provide greaterstart torque than would be provided by run capacitor CR acting alone.Output of gate G1 drops low to initiate compressor starting. In additionto causing compressor motor M energization, the output of G1 causescurrent to be established in resistor R31 and the emitter base of Q10.This action raises the voltage at input pin 8 of gate G4, therebycausing the output of gate G4 to drop low, turning on TRIAC3. TRIAC3turns on TRIAC1 via resistor R8 thereby providing a conductive paththrough the main terminals of TRIAC1. The main terminals of TRIAC1 inturn connect start components capacitor CS and resistor RS across runcapacitor CR to provide an enhancement of start torque. The timeduration of activation of start TRIAC1 and TRIAC3 is determined by theRC time constant which is the product of the values of capacitor C7 andresistor R33. Resistor R32 forms a discharge path for capacitor C7 uponmotor deenergization.

FIG. 6 shows an alternative embodiment of the compressor motor switchingarrangement. Output stage 31' is directly driven by the output of pin 11of NAND gate G3, and does not require oscillator 30 or any of itsrelated circuitry. The output of gate G3 is connected through resistorR71 to the gate of TRIAC5. TRIAC5 controls the activation of relay 33comprising coil 35 and contacts 37, and conduction of the DC currentfrom the 14.5 V supply into the gate terminal of TRIAC5 is thuscontrolled by the output of NAND gate G3. Coil 35 is activated byconduction of TRIAC5 which closes contacts 37 to activate compressormotor M. The embodiment of FIG. 6 provides current isolation between thecontrol circuitry and the power switching circuitry. A suitable relayfor output stage 31' is P.&B. KRPA5AG120. Relay 33 may also be a doublepole type relay, in which case a contact is connected to each of the twomotor terminals. This forms a double break connection, and the relayused for this type connection is commonly called a contactor. Usingoutput stage 31', the circuitry of FIG. 2A would not use transistors Q3,Q8, Q9; resistors R4, R25; capacitors C5, C6; transformer T1; or SCR1 orSCR2. Also, resistor R24 is then connected to the base of transistor Q7.

Fan controller 27 is composed of transistors Q20-Q22, resistors R42-R47,capacitors C10-C11, and NAND gate G5. Fan controller 27 controls the"on" time of the fan via capacitor C10 and resistor R47. The "off" timeof the fan is regulated by capacitor C10 and the series combination ofresistors R46 and R47. Typically, the "on" time for fan operation isabout two minutes, while the "off" time is about eight minutes. Fan 9 isactivated by fan activation circuit 32 comprising NAND gate G6,resistors R48 and R27, transistor Q23, and TRIAC2.

The fan timer circuit is adapted to accept commands from a mode selectorswitch connected at the two terminals of diode D15 to allowfan-on/compressor-off operation, fan-on-timer/compressor-off operation,and fan-on/compressor-on operation. Resistors R45-R47, capacitor C12,and transistor Q22 are configured to monitor both compressor activityand mode selection.

Although the fan control circuitry is disclosed as including a specificarrangement of discrete components, other arrangements may be used,including microprocessor control with preprogrammed software orfirmware. The fan control circuitry includes protection circuitry whichdeactivates both fan 9 and compressor motor M when a fault condition issensed in fan 9.

The present invention may be practiced by using the following values forthe circuit elements described above:

    ______________________________________                                        Label         Value                                                           ______________________________________                                        R1            1 MΩ                                                      R2            PTC Thermistor                                                  R3            470 KΩ                                                    R4            33 KΩ                                                     R5            10 KΩ                                                     R6            10 MΩ                                                     R7            150 KΩ                                                    R8            2.7 KΩ                                                    R9            270 KΩ                                                    R10           Selected (120 KΩ, for example)                            R11           100 KΩ                                                    R12           10 KΩ                                                     R14           1 MΩ                                                      R15           22 Ω                                                      R16           33 KΩ                                                     R17           33 KΩ                                                     R18           33 KΩ                                                     R19           NTC Thermistor                                                  R20           Potentiometer                                                   R23           120 KΩ                                                    R24           270 KΩ                                                    R25           47 Ω                                                      R26           50 KΩ                                                     R27           2.7 KΩ                                                    R28           2.7 KΩ                                                    R29           2.7 KΩ                                                    R31           1 MΩ                                                      R32           4.7 MΩ                                                    R33           4.7 MΩ                                                    R35           820 Ω                                                     R41           820 Ω                                                     R42           100 KΩ                                                    R43           10 KΩ                                                     R44           100 KΩ                                                    R45           10 KΩ                                                     R46           22 MΩ                                                     R47           10 MΩ                                                     R48           33 KΩ                                                     R49           NTC Thermistor                                                  R50           5.1 KΩ                                                    R51           100 KΩ                                                    R52           100 KΩ                                                    R60           3.0 MΩ                                                    R61           100 KΩ                                                    R62           24 KΩ                                                     R63           2.7 KΩ                                                    R64           33 KΩ                                                     R65           8.2 KΩ                                                    R66           820 Ω                                                     R67           3.9 Ω                                                     R68           22 KΩ                                                     R69           22 KΩ                                                     R70           22 MΩ                                                     R71           2.7 KΩ                                                    RS            5.0 Ω, 10 Watt                                            C2            2.2 μƒ                                              C3            15 μƒ                                               C4            2.2 μƒ                                              C5            0.1 μƒ                                              C6            47 pƒ                                                  C7            0.1 μƒ                                              C8            0.1 μƒ                                              C9            470 μƒ                                              C10           15 μƒ                                               C11           0.1 μƒ                                              C12           0.1 μƒ                                              C13           0.1 μƒ                                              C14           0.1 μƒ                                              C15           2.2 μƒ                                              CS            100 μƒ                                              CR            15 μƒ                                               D1            28 V, 1/2 w Zener                                               D2            IN4148                                                          D4            15 V, 1 w Zener                                                 D5            IN4004                                                          D6            IN4004                                                          D9            IN4001                                                          D11           IN4148                                                          D12           IN4148                                                          D14           IN4148                                                          D16           IN4148                                                          Q3            2N3906                                                          Q5            2N3906                                                          Q6            2N3904                                                          Q7            2N3904                                                          Q8            2N3904                                                          Q9            2N3904                                                          Q10           2N3906                                                          Q11           2N3906                                                          Q12           2N3904                                                          Q20           2N3906                                                          Q21           2N3904                                                          Q22           2N3904                                                          Q23           2N3906                                                          Q60           2N3904                                                          Q61           2N3904                                                          Q62           2N3904                                                          G1            CD4093BE                                                        G2            CD4093BE                                                        G3            CD4093BE                                                        G4            CD4093BE                                                        G5            CE4093BE                                                        G6            CE4093BE                                                        G7            CE4093BE                                                        G8            CE4093BE                                                        SCR1          MCR225-6FP                                                      SCR2          MCR225-6FP                                                      TRIAC1        T2500M                                                          TRIAC2        2N6073B                                                         TRIAC3        MAC97B                                                          TRIAC4        MAC223-6FP                                                      TRIAC5        2N6073B                                                         ______________________________________                                    

It should be understood that the signals generated by the circuitry ofthe present invention may take many forms, such as voltage levels asdisclosed, logic levels, polarity, current levels, etc.

While this invention has been described as having a preferred design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

What is claimed is:
 1. A refrigeration system for cooling a chamber,said refrigeration system comprising:a compressor; a thermostat disposedin the chamber to be cooled; a heat exchange apparatus coupled to saidcompressor and in thermal communication with the chamber to be cooled; afan having a motor, said fan disposed in fluid communication with saidheat exchange apparatus; and a control circuit coupled to saidcompressor, said fan, and said thermostat, said control circuit havingfan control circuitry selectively activating said compressor and saidfan in said response to said thermostat to cool the chamber, said fancontrol circuitry including a gate and a solid state switch, said solidstate switch being responsive to said gate, and said gate beingresponsive to a second gate, said control circuit also includingprotection circuitry for sensing a fault condition in said fan, saidprotection circuitry including said second gate and a thermistor inthermal contact with said fan motor, said second gate being responsiveto said thermistor, said protection circuitry deactivating saidcompressor and said fan when a fault condition is sensed in said fan. 2.The refrigeration system of claim 1 wherein said control circuitincludes a fan timer which selectively activates said fan.
 3. Therefrigeration system of claim 2 wherein said control circuit operatessaid fan whenever said compressor is operating regardless of said fantimer.
 4. The refrigeration system of claim 1 wherein said fan includesa motor, and said protection circuitry further includes a resistanceresponsive to the temperature of said fan motor, said resistance beingcoupled to said control circuit for de-activating said compressor whensaid fan motor temperature exceeds a predetermined level.
 5. Therefrigeration system of claim 1 wherein said control circuit includes asolid state switch for activating and deactivating said fan.
 6. Therefrigeration system of claim 1 wherein said control circuit includes amode selector switch providing for at least three modes of operation, afirst mode for operating only said fan, a second mode for operating saidfan on timing cycles when said compressor is not operating and runningsaid fan when said compressor is operating, and a third mode foroperating said fan continuously, said control circuit utilizing one ofsaid first, second, and third modes according to said mode selectorswitch.
 7. The refrigeration system of claim 1 further comprising asystem switch for selectively connecting said compressor to a powersupply, wherein said control circuit include motor deactivationcircuitry which delays the starting of said compressor for apredetermined time period after said system switch connects saidcompressor to the power supply subsequent to deactivation of said motorwhen a fault condition is observed, said control circuit furtherincluding a user activated switch connected to over-ride circuitry whichimmediately initiates operation of said compressor motor incontravention of said motor deactivation circuitry.
 8. The refrigerationsystem of claim 1 wherein said over-ride circuitry only initiatesoperation of said compressor motor for a predetermined number ofattempts, and if said compressor motor does not attain a runningcondition then said over-ride circuitry locks out operation of saidcompressor motor for a second predetermined time period.
 9. Therefrigeration system of claim 7 further comprising start-relay means fordecreasing the impedance of circuit elements serially connected withsaid compressor motor for a predetermined amount of time, wherebydecreasing the impedance of said circuit elements increases the torqueproduced by said compressor motor.
 10. The refrigeration system of claim1 wherein said control circuit includes a solid state switch forelectrically coupling said compressor and a power supply.
 11. Therefrigeration system of claim 10 wherein said control circuit includesan inductive coupling adapted to induce a current on the activation gateof said solid state switch and thereby provide power to said compressor.12. A refrigeration system for cooling a chamber, said refrigerationsystem comprising:a compressor; a thermostat disposed in the chamber tobe cooled; a heat exchange apparatus coupled to said compressor and inthermal communication with the chamber to be cooled; a fan having amotor, said fan disposed in fluid communication with said heat exchangeapparatus; a control circuit coupled to said compressor, said fan, andsaid thermostat, said control circuit selectively activating saidcompressor and said fan in said response to said thermostat to cool thechamber, said control circuit including protection circuitry for sensinga fault condition in said fan, said protection circuitry deactivatingsaid compressor and said fan when a fault condition is sensed in saidfan; and a system switch for selectively connecting said compressor to apower supply, said control circuit includes motor deactivation circuitrywhich delays the starting of said compressor for a predetermined timeperiod after said system switch connects said compressor to the powersupply subsequent to deactivation of said motor when a fault conditionis observed, said control circuit further including a user activatedswitch connected to over-ride circuitry which immediately initiatesoperation of said compressor motor in contravention of said motordeactivation circuitry.
 13. The refrigeration system of claim 12 whereinsaid over-ride circuitry only initiates operation of said compressormotor for a predetermined number of attempts, and if said compressormotor does not attain a running condition then said over-ride circuitrylocks out operation of said compressor motor for a second predeterminedtime period.
 14. The refrigeration system of claim 12 further comprisingstart-relay means for decreasing the impedance of circuit elementsserially connected with said compressor motor for a predetermined amountof time, whereby decreasing the impedance of said circuit elementsincreases the torque produced by said compressor motor.